1. Field of the Invention
This invention relates generally to mobile communication and more particularly to the determination of the positions of base stations in a mobile communication network.
2. Description of the Related Art
Mobile communication networks are in the process of offering increasingly sophisticated capabilities for locating the position of a mobile terminal of the network. The regulatory requirements of a jurisdiction may require a network operator to report the location of a mobile terminal when the mobile terminal places a call to an emergency service, such as a 911 call in the United States. In a Code Division Multiple Access (CDMA) digital cellular network, the position location capability can be provided by Advanced Forward Link Trilateration (AFLT), a technique that computes the location of the mobile station (MS) from the mobile station's measured time of arrival of radio signals from the base stations. A more advanced technique is hybrid position location, where the mobile station employs a Global Positioning System (GPS) receiver and the position is computed based on both AFLT and GPS measurements.
Message protocols and formats for CDMA position location employing AFLT, GPS, and hybrid receivers, applicable to both the MS-based and MS-assisted cases, have been published in TIA/EIA standard IS-801-1 2001, Position Determination Service Standard for Dual-Mode Spread Spectrum Systems—Addendum, incorporated herein by reference. Page 4-43 of this standard specifies that each base station shall transmit a GPS reference time correction of the base station antenna transmitting the CDMA pilot pseudorandom (PN) sequence.
Another position location technique is where the measurements are made by a network entity, rather than the mobile station. An example of these network-based methods is the RTD measurement carried out by the serving base stations. Measurements made by the mobile station may be combined with network-based measurements to enhance the availability and accuracy of the computed position.
Data concerning calibration or re-calibration of a base station time offset, base station antenna location and other parameters are stored in what is called a “base station almanac.” The base station almanac data base provides information for determining an initial position estimate to seed GPS pseudorange searching. The base station almanac data base provides information for resolving ambiguity about which observed pseudorandom noise sequences (PNs) equate to which physical sectors of a GPS-capable IS-95 CDMA network. The base station almanac data base provides the cellular base station sector antenna position from which signals emerge. AFLT range measurements are made to these antenna positions.